Photon correlation spectroscopy (also termed dynamic light scattering, or DLS) is a technique for characterising particles by the temporal variation in the intensity of light scattered from a region of the sample. A time series of measurements of scattered light is used to determine a size or size distribution of particles dispersed in the sample.
As discussed in WO2009/090562, it is well known that the intensity of light scattered by particles smaller than the wavelength of the illuminating light is a strong function of particle size. In the Rayleigh scattering limit, where particle radius is below 0.1 of the wavelength of the illuminating light, the intensity of scattered light is proportional to the sixth power of the particle radius. The light scattered from such small particles is also substantially isotropic. Therefore in a dispersion of proteins, typically of size 0.3 nm-20 nm, an aggregate or filter spoil particle, e.g. >100 nm in size, may dominate the signal until it has diffused away from the optical detection volume within the sample. In the often used Cumulants reduction, the output of Z average and polydispersity index (Pdi), may be badly skewed by the larger fraction.
This sensitivity to ‘dust’ is known, with many literature sources stressing the importance of careful sample preparation, however the presence of filter spoil or aggregates is difficult to avoid completely.
A light scattering measurement on a sample containing primarily small particles and also larger particles can be very sensitive to the larger particles, or even to individual large particles. The larger particles can degrade the quality with which the smaller particles can be characterised. Such larger particles may be unwanted contaminants: they may be aggregates of the primary particles, or some other material.
WO2009/090562 proposes addressing this problem by use of multiple photon counting detectors. A supplemental detector at a low scattering angle is proposed to detect when a larger particle is scattering light, and then data from a detector intended for DLS analysis can be ignored when larger particles are present.
Although this represents a significant advance, shortcomings still remain, and an improved method and apparatus for DLS is desirable.
It is also known to perform particle characterisation by analysing a pattern of diffracted/scattered light from a sample. The light source is generally a laser, and this type of analysis may sometimes be referred to as laser diffraction analysis or Static Light Scattering (SLS). Large particles may also be a problem in static light scattering measurements: scattering from larger particles may obscure relatively small amounts of light scattered from smaller particles.
To the extent that prior art methods consider the problem of contaminants, data that includes scattering from contaminants is simply discarded. The consequence of this crude approach is that data may be wrongly discarded, and as a consequence incomplete results may be presented, or longer run times may be necessary to obtain sufficient valid data.
Methods and apparatus that facilitate reliable analysis of polydisperse particles are desirable.
According to a first aspect of the invention, there is provided a method of characterising particles in a sample, comprising: illuminating the sample in a sample cell with a light beam, so as to produce scattered light by the interaction of the light beam with the sample; obtaining a time series of measurements of the scattered light from a single detector; determining, from the time series of measurements from the single detector, which measurements were taken at times when a large particle was contributing to the scattered light; and determining a particle size distribution from the time series of measurements, including correcting for light scattered by the large particle.